Why Space Tethers have a Bad Reputation

A number of rather ambitious and far-out tether concepts have been proposed
by other people.
Many of these concepts require materials and technology
well beyond what we have today. These have given tethers in general
the reputation of
not being practical today.

A space elevator passes through the atmosphere, unlike a space tether.
If it is hit by lightning it will be destroyed.
Nanotubes conduct electricity well. A conductor going all
the way through the atmosphere would be the ultimate lightning rod.
The space elevator
people say they will put it where there is not much lightning.
The investment in dollars and the risk to humans riding the elevator
makes even a chance of lightning a serious issue.
A space tether does not have a lightning problem at all.

A geo-stationary tether station is about 36,000 km away,
and you have to climb uphill the whole way.
If the climber averages 100 km/hour it takes 15 days to climb up.
Solar powered cars can do this kind of speed on flat ground, but straight up
it is not easy. At this rate it would take a long time to bring up cargo
equal to
the initial weight of the tether (and this assumes you had materials we
don't have so that you could even build it). Some places
advocate beaming power to the elevator to get around this.
Even with plenty of power, there are limits to how fast wheels can climb a rope.
And this beaming power is a futeristic concept that makes it seem like tethers
are not practical today. In fact, a
NASA study estimates it will be 50 to 100 years before this type is practical.
This is not what is called "near term technology".

The reliability of elevator climber is a problem. I doubt
that any car I have owned has ever gone 22,000 miles without
needed some repair, and I am mostly going on flat ground.
To go straight up for 22,000 miles without having to have
a mechanic fix something is really high reliability for a
wheeled vehicle. So this will be a high priced vehicle
since high reliability takes high priced engineering
and high priced materials. A rotating tether does not need a climber.

A rotating tether is
practical with with existing materials and technology.
It needs much less mass and so it is cheaper to build and launch.
A rotating tether can also lift payloads much faster (like 10 minutes)
and more often. Using cheap existing ropes, less mass in space
to get started, and more frequent payloads, makes them far more
financially interesting.

If it takes 15 days to climb, you have a radiation risk.
You are passing through the Van Allen belts. This is not
a good place to stay for days even with a couple centimeters
of shielding. Your GEO hotel/space-station is going to
have plenty of shielding and is only on the far edge of
the Van Allen belts. Taking a long time to get there
also increases your chance of getting caught by a
a solar partical event (aka solar flare).
A rotovator can toss to GEO in 4 hours, which
greatly reduces the radiation risk. The space elevator approach
would require a lot more shielding to get the radiation risk down
to the level of the rotovator. The shielding is dead weight.
It could take several times the mass of the people to
shield the people, which raises the cost of the elavator
approach.

The Van Allen belts are also going to be hard on the
nanotubes used to make the Space Elevator. The radiation
will weaken them rapidly. A rotating space tether does
not need to be in the Van Allen belts.

The kind of rotating tether SpaceTethers.com advocates
can grab a payload and then
toss it about 10 minutes later. With one launch site we could do a payload
every orbit, or about every 100 minutes. This is like 14 times per day,
instead of every few days as a Space Elevator would be.

The main advantage of space elevators over rotating tethers
seems to be that it is easier to explain to someone.

Tether Failures

There have been a few widely publicized tether experiment failures. This gives
tethers the image of not working. The experiments that work are not as
newsworthy. In the early days of airplanes, where we did not understand
so much about flying, there were many newsworthy failures too. It is still
the early days of tethers, but understanding and reliability are not far off.

Not All Tethers Are Bad

There are some types of tethers that could be built today. The design proposed
by SpaceTethers is small enough to be sent up in one commercial rocket launch.
This is a very practical design because it starts out small and
uses only off the shelf materials.

Note that at least one tether has actually already been in orbit for years without
breaking.

Links

A good paper on the costs/Kg of launching with a Space Elevator is
Launch Systems Cost by Jordin T. Kare .
The development cost of the Space Elevator is so high that if you
take capital costs into account your costs/Kg may not be cheaper
than current launch vehicles, let alone cheaper than what they can
be in 20 years. The super strong carbon-nanotube composite that is
needed for space elevators would make pressure tanks for rockets
much lighter. Probably years before it is possible to make enough
for a space elevator it will be possible to make enough for
some rocket tanks.